1. Field of the Invention
Implementations of various technologies described herein generally relate to seismic data processing, and more particularly, processing seismic data acquired using twin over/under streamers.
2. Description of the Related Art
The following descriptions and examples do not constitute an admission as prior art by virtue of their inclusion within this section.
Seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. Since many commercially valuable hydrocarbon deposits are located beneath bodies of water, various types of marine seismic surveys have been developed. In a typical marine seismic survey, seismic streamers are towed behind a survey vessel. The seismic streamers may be several thousand meters long and contain a large number of sensors, such as hydrophones, geophones, and associated electronic equipment, which are distributed along the length of the seismic streamer cable. The survey vessel may also include one or more seismic sources, such as air guns and the like.
The seismic streamers may be in an over/under configuration, i.e., one set of streamers being suspended above another set of streamers. Two streamers in an over/under configuration, referred to as twin streamers, may be towed much deeper than streamers in a conventional single configuration.
As the seismic streamers are towed behind the survey vessel, acoustic signals, commonly referred to as “shots,” produced by the one or more seismic sources are directed down through the water into strata beneath the water bottom, where they are reflected from the various subterranean geological formations. Reflected signals are received by the sensors, digitized, and then transmitted to the survey vessel. The digitized signals are referred to as seismograms and are recorded and at least partially processed by a signal processing unit deployed on the survey vessel. The ultimate aim of this process is to build a representation of the subterranean geological formations beneath the streamers. Analysis of the representation may indicate probable locations of hydrocarbon deposits in the subterranean geological formations.
In order to build the representation of the subterranean geological formations, streamer seismic data may be processed in an effort to create an up-going wave field at the sea surface, or zero depth. Typically, the seismic data from a streamer has notches or areas without data because the equipment used may be insensitive at certain frequencies. A primary aim of twin streamer acquisition is to use the second streamer data to fill in the information at the notch frequencies of the first streamer. In addition, the seismic data from each streamer may be a combination of up-going and down-going wave fields. The up-going wave fields may include data about the subterranean geological formations. The down-going wave fields may typically be reflections of the up-going wave fields at the sea surface, referred to as surface seismic ghosts. The down-going wave fields may be considered noise that needs to be attenuated. A multitude of processing techniques generally aimed at filling the notch frequencies and removing the down-going wave fields have been developed.
Various conventional techniques for processing seismic data acquired by streamers or sources in a twin over/under configuration may use a shift and subtract algorithm. Generally in a shift and subtract algorithm, the data recorded on one of the twin over/under streamers may be time shifted such that the up-going or down-going wave fields occur at the same time on both streamers. Subtraction of one data set from the other may remove the aligned wave fields. The data set of the wave fields may be commonly referred to as a ghost and may be dependent on the vertical separation of the twin over/under streamers.
Other conventional techniques for processing seismic data acquired by streamers or sources in a twin over/under configuration involve a dephase and sum algorithm. Generally in the application of the dephase and sum algorithm, the data recorded at each streamer may be first processed by a correlation step to dephase the individual surface ghost wavelets for each of the two streamers. The two dephased data sets may then be summed yielding a seismic data set that would have been acquired by a single streamer at the sea surface.